Developing an equivalent toxicity area approach to comparing toxicity of urban road deposited sediments

Spatio-temporal variability of potentially toxic elements' pollution in road-deposited sediments according to health risk thresholds: a meta-analysis

Road deposited sediments (RDS) are important sinks of potentially toxic elements (PTEs), which may have a significant impact on human health. A systematic review of published papers on the PTEs occurrence in RDS was carried out. The main goal was to assess the global RDS contamination by PTEs and human health risks linked with anthropogenic activities. A systematic search was made to collect information about the most cited PTEs in the published literature and perform a statistical analysis. Subsequently, health risks were assessed for 35 different areas worldwide. PTE concentrations showed high variability, and means were multiple times higher than the corresponding consensus-based threshold effect concentrations (5.2-, 10.3-, 5.3-, 3-, 7.3-, and 3.6-fold higher for Zn, Pb, Ni, Cr, Cu, and Cd, respectively). PTEs concentrations were ranked as Zn > Pb > Cu > Mn > Cr > Ni > Cd. Non carcinogenic risks followed the trend Pb > Cu > Zn > Cd. Lead is responsible for the highest significant non carcinogenic risk to human health. Unacceptable exposition to carcinogenic risks is present in most areas. The top carcinogenic risk areas were Singapore > Beijing > Yixing > Shanghai > Zhuzhou for adult male, Dresden > Singapore > Ulsan > Huludao for adult females, and Dresden > Singapore > Ulsan > Huludao for children. Highest chromium and nickel carcinogenic risks occurred in Singapore, Cd in Dresden, and Cu in Huludao. Highest RDS contamination was seen in industrial areas due to pollutants deposition. Highest Zn, Cu, Cd, and Pb concentrations occur in densely urbanized areas due to heavy-duty vehicular exhausts.

Toxicity of urban stormwater on Chlorella pyrenoidosa: Implications for reuse safety

Urban stormwater is an alternative water source used to mitigate water resource shortages, and ensuring the safety of stormwater reuse is essential. An in-depth understanding of both individual pollutant concentrations/loads in stormwater and holistic stormwater quality can be used to comprehensively evaluate how safely stormwater can be reused. The toxicity test takes all pollutants present in water samples into account, and the results reflect the integrated effect of these pollutants. In this study, the influence of urban stormwater sourced from different land uses on microalgae (Chlorella pyrenoidosa) and the possible toxicity mechanisms were investigated. The results showed that urban stormwater, particularly residential road stormwater, significantly inhibited microalgal growth. The chlorophyll contents of microalgae exposed to residential road stormwater were relatively lower, while the corresponding values were relatively higher for microalgae exposed to grassland road stormwater. Additionally, the antioxidant-related metabolism of microalgae could be dysregulated due to exposure to urban stormwater. A possible toxicity mechanism is that urban stormwater influences metabolic pathways related to chlorophyll synthesis and further hinders photosynthesis and hence microalgal growth. To resist oxidative stress and maintain regular microalgal cell activities, the ribosome metabolism pathway was upregulated. The research results contribute to elucidating the toxicity effects of urban stormwater and hence provide useful insight for ensuring the safety of stormwater reuse. It is also worth noting that the study outcomes can only represent the influence of land use on stormwater toxicity, while the impacts of other factors (particularly rainfall-runoff characteristics) have not been considered. Therefore, the consideration of all influential factors of stormwater is strongly recommended to generate more robust results in the future and provide more effective guidance for real practices related to stormwater reuse safety.

Characterizing polycyclic aromatic hydrocarbons on road dusts in Shenzhen, China: implications for road stormwater reuse safety

Urban road stormwater reuse is one of the most effective ways to mitigate water resource shortage. However, due to a diversity of human activities such as traffic, various toxic pollutants can be deposited on road surfaces during dry periods and washed off during wet periods, threatening stormwater reuse safety. Among these pollutants, polycyclic aromatic hydrocarbons (PAHs) have been widely found in road stormwater. This study selected twelve road sites in Shenzhen, China, and investigated PAHs deposited on urban roads and their influential factors (traffic characteristics, land use and road surface condition). The research outcomes showed that high-molecular-weight PAH species (5-6 benzene rings) had higher concentrations and variability on spatial distributions than light-molecular-weight ones (2-4 benzene rings). Additionally, more PAHs were attached to dusts with small particle sizes (< 150 µm), and among influential factors, commercial land use showed a stronger correlation with PAHs distributions, regardless of particle sizes. Furthermore, it is noteworthy that traffic volume did not have an important influence on PAH generations on roads, while the source tracking results did indicate that traffic activities were the main contributor of PAHs. This implies that other traffic characteristics such as frequent go-and-stop activities might also contribute PAHs on roads. This means that areas with frequent traffic congestions could be the "hot spot" areas of PAHs, although the traffic volume might be not high. These research outcomes can provide useful insight into effective stormwater management and ensuring their reuse safety.

Leaching risks of antibiotic resistance genes in urban underlying surface sediments during the simulated stormwater runoff and its controls

Recently, increasing attention has been paid to antibiotic resistance genes (ARGs) in urban stormwater runoff. However, no available literature could be found on ARGs leaching from urban underlying surface sediments during stormwater runoff. In this study, surface sediments from commercial public squares around Nanjing (China) was selected for the investigation of target ARGs leaching kinetics, showing that absolute abundances of target ARGs desorption reached at the equilibrium during leaching time of 120-240min with all of the desorption efficiencies about 30%, indicating that there would be low proportion of leaching ARGs in the total ARGs migrating with runoff during rainfall events. Five target ARGs leaching including intI1 (clinic), strA, strB, tetM and tetX can be better described by the pseudo-second-order equation, while qacEdelta1 leaching can be better described by the pseudo-first-order equation, and the leaching for both sul1 and sul2 can be well described by the pseudo-first-order and pseudo-second-order equations. The effects of environmental factors including S/L ratios, pH values and water temperatures indicated that leaching efficiencies of target ARGs enhanced significantly with the increase of S/L ratios and water temperatures, but decreased with the increase of pH values. The transmission experiments after the simulated solar irradiation and heat implied that both large solar UV irradiation (30W/m²) and high temperature (40℃) were conducive to conjugation and transformation frequencies of ARGs. Furthermore, both high Cao and medium PAM levels could be effective for blocking ARGs transmission in the leachate from underlying surface sediments.

Integrating Tank Model and adsorption/desorption characteristics of filter media to simulate outflow water quantity and quality of a bioretention basin: A case study of biochar-based bioretention basin

Reliable approaches for accurately assessing the performance of stormwater treatment systems is essential for their effective design, including filter media selection which can be a significant constituent in stormwater treatment systems. This study presents an innovative modelling approach integrating the Tank Model with the adsorption-desorption characteristics of the filter media. The resulting modelling approach was applied to simulate a field-scale bioretention basin where biochar was used as filter media with over ten years of rainfall records. The resulting outflow and overflow volumes were compared with observed data for calibration. The Stormwater Treatment Tank Model (STTM) was validated using the Leave-One-Out-Cross-Validation (LOOCV) method. The simulation outcomes include water outflow and overflow (quantity) from the bioretention basin as well as outflow water quality represented by three heavy metals (Pb, Cu, and Zn). The modelling approach developed was found to be capable of accurately simulating outflow and overflow volumes, with outlet water quantity being significantly influenced by the total rainfall depth. The modeling results also suggested that a sole treatment system would not be adequate, particularly for large rainfall events (>100 mm) and a treatment train would be more effective. Simulating long-term (over ten years) pollutant removal performance in the bioretention basin indicated that heavy metals outflow event mean concentration (EMCs) values calculated using simulated results of 30% biochar application rate generated the best pollutant removal with consistent values (2.7 μg/L, 3.0 μg/L, 17.2 μg/L for Pb, Cu, and Zn, respectively). These results confirm that the modelling approach is reliable for assessing long-term treatment performance, as well as a robust tool able to contribute to more effective treatment system design, particularly filter media selection and evaluation.

Toxicity variability of urban road stormwater during storage processes in Shenzhen, China: Identification of primary toxicity contributors and implications for reuse safety

Urban road stormwater reuse is one of the most important ways to mitigate water resource shortage. Generally, stormwater is stored prior to reuse or further treatment. This study explored the stormwater toxicity variability during two types of storages, closed and open storages using Chinese hamster ovary cells (CHO), which are mammalian cells. The toxicity test by CHO cells can indirectly represent the risk related to human health. Both rainfall (without reaching ground surfaces) and urban road stormwater were collected to undertake laboratory-scaled storage experiments and basic water quality parameters (pH and dissolved oxygen), microorganisms (E.coli and total bacteria), total organic carbon and heavy metals (copper, Cu, zinc, Zn, nickel, Ni, chromium, Cr, cadmium, Cd and lead, Pb) were also investigated during storage processes. The outcomes showed that rainfall has a better water quality with lower toxicity than urban road stormwater (EC₅₀ values of rainfall were generally twice higher than road stormwater). Additionally, it is found that storing road stormwater for a certain period would reduce the toxicity and hence improve their reuse safety (EC₅₀ values in Day 1 were 10.30 mL and 8.46 mL for closed and open storage respectively while they were 14.3 mL and 13.0 mL in Day 7). Organic matters and Cu are important contributors of toxicity during both closed and open storages while bacteria is also essential in toxicity contribution in open storage. The research results implied that storing stormwater for a certain period has a benefit for reuse safety. This is related to cost-effectiveness in terms of treatment system design to avoid over engineering. Additionally, it is suggested that for reducing toxicity, the stormwater treatment designed before/after storage devices should focus on removal of organic matters and heavy metals (specially Cu) as well as restraining bacteria growth.

Comparison of pollutant source tracking approaches: Heavy metals deposited on urban road surfaces as a case study

A range of source tracking approaches have been developed to identify sources in the environmental pollution research field. A comparison of source tracking approaches is essential for a better understanding and practical applications of these approaches. This study compared the commonly used source tracking approaches, namely positive matrix factorization (PMF), Unmix, flag element ratio (FER), and chemical mass-balance based stochastic approach (SCMD). A case study was illustrated for tracing heavy metals (Pb, Zn, Cr, Cu, and Ni) attached to road deposited sediments, which can significantly influence urban road stormwater quality. The results indicated that the accuracy of PMF and Unmix are affected by the number of chemical species used and whether useful markers can be identified for particular sources. However, this does not have an essential influence on FER and SCMD. PMF and Unmix are easier on data preparation and calculation processes but more difficult for source identification process than FER and SCMD. This study also provided recommendations related to the selection of source tracking approach based on different study scenarios and result requirements. These study results are able to provide important guidance for undertaking effective source tracking and devising environmental pollution mitigations.